1. Field of the Invention
The present invention relates to an apparatus and a method for driving a liquid crystal display device (hereinafter, referred to as an "LCD device"), and in particular to an apparatus and a method for driving a liquid crystal panel using a ferroelectric liquid crystal material (hereinafter, referred to as an "FLC material") having a negative dielectric anisotropy.
2. Description of the Related Art
FLC materials are today actively studied for use in an LCD apparatus having a large display capacity with high precision due to excellent characteristics such as memory capability, high-speed response and a wide viewing angle.
FIG. 2 is a cross sectional view illustrating a basic structure of an FLC panel 1. Such a basic structure is common to FLC panels used in the present invention.
As is shown in FIG. 2, the FLC panel 1 includes two glass substrates 2a and 2b opposed to each other. On a surface of the substrate 2a, a plurality of signal electrodes S formed of a transparent material such as indium tin oxide (hereinafter, referred to as "ITO") are provided parallel to each other. An insulation film 3a formed of SiO.sub.2 or the like is provided on the substrate 2a so as to cover the signal electrodes S. On a surface of the other substrate 2b, a plurality of transparent scanning electrodes L formed of ITO or the like are provided so as to be opposed to and perpendicular to the signal electrodes S. An insulation film 3b formed of SiO.sub.2 is provided on the substrate 2b so as to cover the scanning electrodes L. Alignment films 4a and 4b are provided on the insulation films 3a and 3b, respectively. The alignment films 4a and 4b are formed of an organic polymer such as polyimide, nylon or polyvinyl alcohol or a film obtained by oblique evaporation of SiO, and are treated to align liquid crystal molecules in one direction.
The glass substrates 2a and 2b are assembled together with a sealing agent 5, and a gap between the insulation films 4a and 4b is filled with an FLC material 6 which is injected through an opening. The opening is sealed with the sealing agent 5 after the injection of the FLC material 6. The glass substrates 2a and 2b are interposed between a pair of polarizing plates 7a and 7b, which are arranged so that polarizing axes thereof be perpendicular to each other.
As is shown in FIG. 3B, an FLC molecule 10 has spontaneous polarization Ps in a direction perpendicular to the longer axis thereof. The FLC molecule 10 is supplied with a force which is in proportion to a vector product of an electric field E and the spontaneous polarization Ps, and thus moves on an upper part of a peripheral surface of a cone 11 having an apical angle which is twice a tilt angle .theta.. The electric field E is generated by a voltage applied to the signal electrodes S and a voltage applied to the scanning electrodes L.
At this point, the FLC molecule 10 is provided with a force F which is in proportion to a dielectric anisotropy .DELTA..epsilon. and a square of the electric field E. The dielectric anisotropy .DELTA..epsilon. is the difference between the dielectric constant in the direction of the longer axis and that in the direction of the shorter axis. The force F is represented by the following equation. EQU F=K.sub.0 .times.Ps.times.E+K.sub.1 .times..DELTA..epsilon..times.E.sup.2
It is known that, in the case where the dielectric anisotropy (.DELTA..epsilon.) of the FLC material injected into the gap between the insulation films is negative, the force F applied to the FLC molecule 10 has a maximum at a certain level of the electric field. It is also known that, as is described in detail in Japanese Laid-Open Patent Publication No. 64-24234 and the like and is appreciated from FIG. 4 which shows the voltage vs. response time characteristic of the FLC material, the FLC material has a specific voltage (Vmin) at which the response time is minimum.
Methods for driving an FLC panel utilizing such a phenomenon are described in, for example, The "Joers/Alvey" Ferroelectric Multiplexing Scheme, (Ferroelectrics Vol. 122 (1991)), page 63 and International Patent Publication No. WO92/02925.
An FLC panel including an FLC material having a negative dielectric anisotropy driven by one of such methods has, for example, the following advantages over an FLC panel including an FLC material having a positive dielectric anisotropy:
(1) The displayed image has a higher contrast; and PA1 (2) The alignment state of the FLC molecule 10 is stable in a wider temperature range, thus broadening the operation temperature.
Due to such advantages, the FLC panel including an FLC material having a negative dielectric anisotropy is effective for a display apparatus. While the response time is shortened in accordance with increase in the driving voltage in the FLC panel including an FLC material having a positive dielectric anisotropy, the FLC panel including an FLC material having a negative dielectric anisotropy has a specific voltage (Vmin) at which the response time is minimum. Such a specific voltage (Vmin) changes in accordance with the operation temperature or the like. Accordingly, for example, in the case where the operation temperature changes, the driving conditions need to be reset so that the FLC panel has such a specific voltage (Vmin).
FIG. 5 schematically shows a planar structure of a conventional simple matrix FLC display device (hereinafter, referred to as an "FLCD") 20 including the FLC panel 1.
The FLCD 20 operates in the following manner. A voltage is applied by a scanning electrode driving circuit 21 to each of a plurality of scanning electrodes Li (i=0 through 9 and A through F in FIG. 5), and a voltage is applied by a data electrode driving circuit 22 to each of a plurality of signal electrodes Sj (j=0 through 9 and A through F in FIG. 5). A driving voltage corresponding to a potential difference between one of the scanning electrodes Li and one of the corresponding signal electrode Sj is applied to a pixel Aij which is the intersection of the scanning electrode Li and the signal electrode Sj to turn "ON" or "OFF" the display. The data electrode driving circuit 22 includes a transfer circuit 23a for transferring data, a holding circuit 24a for holding the data for a certain time period in accordance with a latch pulse signal (LP), and a voltage generation circuit 25a for generating a voltage in accordance with the data.
The scanning electrode driving circuit 21 includes a transfer circuit 23b for transferring data, a holding circuit 24b for holding the data for a certain time period in accordance with the latch pulse signal (LP), and a voltage generation circuit 25b for generating a voltage in accordance with the data.
The FLC molecule 10 in a pixel Aij and the polarizing axes of the polarizing plates 7a and 7b have the following relationship. The FLC molecules 10 have two stable orientation states when aligned parallel to tilt axes 104 and 105 in FIG. 3A. The tilt axes 104 and 105 are symmetrical to each other with respect to a center line 103. In the case where the polarizing axis of one of the polarizing plates 7a and 7b is perpendicular to the tilt axes 104 or 105 and the polarizing axes of the polarizing plates 7a and 7b are perpendicular to each other (crossed nicols state), a pixel Aij in which the FLC molecules 10 are in one of the stable orientation states is in a dark display state, and a pixel Aij in which the FLC molecules 10 are in the other stable orientation state is in a bright display state.
In this specification, the polarizing axis of the polarizing plate 7a is perpendicular to the tilt axis 104. In this case, a pixel Aij in which the FLC molecules 10 are in the stable orientation state 104 (first stable orientation state) is in a dark display state, and a pixel Aij in which the FLC molecules 10 are in the stable orientation state 105 (second stable orientation state) is in a bright display state. The FLC molecule 10 is stable in the directions of tilt axes 104 and 105 having an angle of 2.omega. therebetween, not in the direction of tilt axes 106 and 107 having an angle of 2.theta. therebetween, because the FLC molecule 10 is tilted with respect to the substrates when being interposed by the substrates.
FIG. 6A shows a LAT (line address time, i.e., time required to scan one line) which is suitable for each operation temperature for driving an FLC panel by a method described in the specification of International Patent Publication No. WO92/02925 with reference to FIG. 5 in the above-mentioned publication. In this method, the Malvaren III waveforms are used. FIG. 6B shows a Malvaren III waveform of selection signal (writing signal) and a Malvaren III waveform of a non-selection signal (non-writing signal). FIGS. 14A and 14B show a LAT suitable for each operation temperature for driving an FLC panel used in a first example (infra) using the Malvaren III waveforms and the memory angle 28. Curves A and B in FIGS. 14A and 14B correspond to curves (A) and (B) in FIG. 6A.
Curve (A) in FIG. 6A shows how a maximum driving pulse width which is required to cause 100% inversion of the liquid crystal molecules changes in accordance with the temperature when the FLC panel is supplied with the selection signal shown in FIG. 6B. In an area on and above curve (A), display is inverted, namely, writing is performed. Curve (A) is also referred to as a switch minimum curve.
Curve (B) in FIG. 6A shows how a maximum driving pulse width by which the liquid crystal molecules are not inverted changes in accordance with the temperature when the FLC panel is supplied with the non-selection signal shown in FIG. 6B. In an area on and below curve (B), display is not inverted. Curve (B) is also referred to as a non-switch maximum curve.
Accordingly, the FLCD can be driven in an area between curves (A) and (B).
FIG. 6D shows an area in which such an FLC panel can be driven by a conventional method in which an image signal obtained at a fixed frame frequency such as a TV image signal is displayed at a fixed frame frequency. Demonstration display is possible at room temperature, but the LAT is excessively long at low temperature. Accordingly, the FLC panel cannot be driven for practical use, and thus the FLCD 20 cannot be used in TVs and the like.
FIG. 6C shows an ideal LAT for each operation temperature. Even if such a driving margin is obtained by future improvement, proper driving of an FLC panel for obtaining a TV image signal requires the LAT (approximately 50 ps or less in the case of the NTSC system) to be shorter than the LAT at the low temperature indicated by point (C).
Under current circumstances, such a high-speed response is obtained at room temperature but not at the lowest level of the operation temperature because the response time of the FLC material increases by 1.5 times each time the temperature lowers by 5.degree. C.
Due to such problems, further improvement in the response time is needed in order to drive a HDTV (high definition TV) having 1000 or more scanning lines.
Memory angle is another characteristic of the FLC material. The memory angle significantly changes in accordance with the operation temperature and the writing time (namely, LAT). Even if the FLC panel is driven at a prescribed LAT corresponding to the temperature, it is impossible to drive the FLC panel while maintaining the memory angle in a wide temperature range (refer to FIGS. 14A and 14B).
Accordingly, the polarizing axis needs to be changed physically in order to keep the polarizing axis of the polarizing plate 7a (FIG. 2) perpendicular to the tilt axis 104 (FIG. 3A). However, in practice the polarizing plates 7a and 7b are pasted on the glass substrates 2a and 2b, and thus it is virtually impossible to move the polarizing axis.